An enormous abundance of byproduct natural gas results during the process of crude oil recovery. Since natural gas is predominantly methane and methane has about twenty times the greenhouse gas potential of carbon dioxide, much of it is flared near the wellhead. Flaring, however, is becoming increasingly environmentally unacceptable.
Methane is a very low-grade fuel that requires significant infrastructure to transport. Transport by high pressure pipeline or the liquefaction, storage and transport of Liquefied Natural Gas (LNG), requires pumps, pipeline, and tanks, and is too costly unless a large quantity of natural gas is available at a single location. As a result of the infrastructure cost, less than 16% of natural gas is traded internationally.
Many bio-processes using microorganisms in submerged cultures (i.e. growing in aqueous media) require oxygen for cell growth and for the conversion of reactant to a cellular product. Bio-processes traditionally use glucose (sugar) as a carbon source. For certain bio-processes methane may be used as the carbon source. Since both oxygen and methane have a low solubility in water, the absorption of gasses into the liquid must occur at a high rate to meet the biological demand of the microorganisms for both oxygen and methane.
Gas sparged bio-reactors have historically been vertical cylindrical tanks having a tank height to tank diameter aspect ratio of 3 to 5. FIG. 1 shows a perspective view of a prior art vertical tank apparatus. Bio-reactor designers have relied on this geometry to minimize plant floor space, to provide higher hydrostatic head for increased gas solubility, and to facilitate exit gas disengagement from the liquid. Solid or liquid carbon sources, traditionally glucose, have high solubility in water. Gaseous carbon sources such as methane and other hydrocarbon gases, however, have low solubility in water and present a challenge to the reactor designer.
When a gaseous hydrocarbon (e.g. methane) is used as the carbon source, a significant fraction of the gaseous hydrocarbon escapes from the liquid medium before it can be consumed by the microorganisms. Typically only as little as thirty percent of the gaseous hydrocarbon is consumed. Oxygen and oxygen enriched air are also relatively insoluble in water. A high percentage of the oxygen also escapes without being consumed. Since carbon dioxide is typically produced by the microorganisms, the effluent gas is a mixture of unconsumed gaseous hydrocarbon, oxygen (or air) and carbon dioxide. It is often both economically and environmentally unacceptable to release unused gaseous hydrocarbon into the atmosphere. Recycle of the gas mixture requires that a gas separator be employed to remove the carbon dioxide, thereby increasing the investment and the operating cost of the process.
Most prior art vertical cylindrical bio-reactors are typically stirred or agitated. In such agitated bio-reactors a gas mixture, such as methane and oxygen, is typically sparged (i.e., bubbled) at the bottom of the reactor. As the gas rises it moves through stirred regions where mechanical agitation finely divides large gas bubbles into smaller ones. This mechanical agitation also serves to thin the liquid boundary layer which surrounds the bubbles, believed to be the major resistance to gas absorption. As the buoyant force moves the gas up, and ultimately out of the liquid, the residence time of the bubble in the liquid limits the amount of gas that each bubble may deliver to the liquid. In such a reactor, the exiting gas still has a significant unconsumed portion of the initial oxygen and methane remaining, typically mixed with gaseous byproducts of the reacted material, such as carbon dioxide. This gas mixture may be recovered, the desired reagent gases being separated from the carbon dioxide and recycled back into the reactor as is taught by U.S. Pat. No. 5,344,766. Such recovery and separation increases the investment and the operating cost of the process.
Loop reactors have also been employed for carrying out microbiological processes. Such reactors are exemplified by DK 170,824, EP 185,407 and EP 418,187. Vertical loop reactors have a long up-flow tube, a long down-flow tube, and two horizontal connecting tubes between the up-flow and down-flow tubes. Such reactors circulate the liquid medium around the loop using a pump. EP 185,407 introduces the gas into the liquid medium at or near the bottom of the up-flow tube and mixes the gas and liquid using one or more static mixers. The unused gas is collected in a separator at the top of the loop, and the liquid is circulated around the loop into the down-flow tube. EP 418,187 introduces the gas into the liquid medium at one or more locations in the down-flow tube (to provide longer gas residence time) and mixes the gas and liquid with static mixers in both the down-flow and the up-flow tubes.
Horizontally oriented stirred cylindrical reactors are also known. Typical is FR 1,438,895, which discloses an air-sparged apparatus for treating wastewater. The cylindrical vessel has an orifice in the form of a slit located at the top of the vessel, parallel to its longitudinal axis. The orifice is filled with water to a height of 0.1 to 0.15 vessel diameters above the top of the vessel. The advantage provided, according to FR 1,438,895, is that the bubbles of air or gas containing oxygen are retained along at least one and preferably several revolutions in the waste waters and injected oxygen is therefore utilized with a significantly better yield than prior art devices. Since the orifice is open to the air along its top face, the vessel cannot retain gas that reaches this orifice. Such a system would be environmentally unacceptable for use in a bio-process using a gaseous hydrocarbon. Another example of a horizontally stirred reactor is U.S. Pat. No. 4,101,384. This patent utilizes a motor driven perforated paddle wheel stirring device to mix the nutrient and gas to maintain a phase state with a relative density of less that 0.3 of that of water. Another example of a horizontally stirred reactor is U.S. Pat. No. 5,151,368, which teaches the rotation of the entire reactor vessel.
A reactor design to efficiently deliver a hydrocarbon such as methane to bacteria to sequester the carbon as biomass is believed to be both environmentally and economically advantageous. Such biomass may be used as animal feed or converted to higher value materials. It is believed that a reactor that eliminates the need for gas recovery, gas separation, and gas recycling would overcome the disadvantage of the prior art.